The invention relates to methods for analyzing samples of porous materials; in particular, the invention can be used for qualitative and quantitative determination of a penetration profile of drilling mud components, and for studying a deterioration of properties in a near-borehole zone of oil/gas-containing formations associated therewith.
The problem of damaging a near-borehole zone of a formation when subjected to penetrated components of a drilling mud (or a flushing fluid) is very important, especially for long horizontal boreholes, because many of them are pumped in the uncased state, i.e., without a cemented and perforated production string.
Drilling muds are complex mixtures of polymers, particles (calcium carbonate, barium sulfate having a size from hundreds of micrometers to less than one micron), clays, and other additives contained in a “carrier” fluid being “a base” of the drilling mud; water, oil, or some synthetic fluid that can act as the carrier fluid.
In the process of drilling under the influence of excessive pressure a filtrate of drilling mud as well as fine particles contained therein, polymers and other components penetrate into a near-borehole zone of a formation and cause significant reduction in the permeability thereof. In addition, an external filter cake comprised of filtered solid particles and other components of the drilling mud is formed on a wall of a borehole.
During the technological procedure of cleaning the borehole (by gradual putting into production), the external filter cake is partially broken while the penetrated components of the drilling mud are partially washed out of the near-borehole zone, and its permeability is partially restored. Nevertheless, a portion of the components remain irreversibly held in a pore space of a rock (adsorption on surfaces of pores, capture in steam restrictions, etc.) which results in an essential difference between an initial permeability and a permeability restored after carrying out the technological cleaning procedure (usually, the restored permeability is not greater than 50 to 70% of the initial permeability).
The conventional laboratory technique for checking a quality of a drilling mud is a filtration experiment for injecting a drilling mud into a core sample followed by back pumping, (i.e., displacement of the penetrated drilling mud with an initial formation fluid). (cf., Longeron D. G., Argillier J., Audibert A. An Integrated Experimental Approach for Evaluating Formation Damage Due to Drilling and Completion Fluids. SPE 30089, or Jiao D., Sharma M. M. Formation Damage Due to Static and Dynamic Filtration of Water-Based Muds. SPE 23823).
Said prior art technique allows measurement only of an integral hydraulic resistance of a core sample (a ratio of a current pressure differential across the core to a current flow rate), the change of which is caused by the growth/destruction dynamic of the external filter cake at an end face of the core and by accumulation/removal of the drilling mud components in the rock.
However, a profile and a distribution of drilling mud components as well as damaged porosity and permeability along the core sample (i.e., along a filtration axis) after pumping of the drilling mud in (or after back pumping) are important information in understanding the formation damage mechanism and to select a respective technique for increasing a wellbore productivity index (to minimize a damage of a bottomhole formation zone). The present parameters are not measured within said traditional procedure of the drilling mud quality check.
To determine said parameters, it is necessary to use additional techniques.